The present invention relates to a variable ratio gear box. In particular, the present invention relates to a variable ratio gear box employing a coplanar reverted gear train loop and a clutch assembly for changing the gear ratio of the gear box.
Conventional variable ratio gear boxes typically include an input shaft, an output shaft, a pinion gear coupled to each shaft, and a cage assembly disposed around the input and output shafts. The cage assembly comprises at least one pair of cluster gears coupled to the pinion gears and mounted on a shaft adjacent to and parallel to the input and output shafts. A clutch assembly selectively couples the cage assembly to the gear box housing to provide a first gear ratio, or to the input or output shaft to provide a second gear ratio. The conventional variable ratio gear box may include a series of cascaded cage assemblies for providing a plurality of gear ratios.
The conventional variable ratio gear box has been widely used in manual incrementally-variable transmissions. However, the manufacture and assembly of such transmissions has been troublesome due, in part, to the difficulty of aligning the teeth of each gear element of the cluster gears to each other. Further, radial alignment errors of the cluster gear elements or their mounting shaft can cause uneven load sharing. Additionally, the cluster gears and pinions are generally manufactured with helical teeth to increase the contact ratio between the cluster gears and opinions. However, the use of helical teeth produces point contact between congruent teeth flanks, thereby subjecting teeth bearing surfaces to high contact stresses.
Attempts have been made to overcome the deficiencies of the conventional variable ratio gear box. For example, Roeder (U.S. Pat. No. 2,005,167) teaches an incrementally-variable transmission comprising coaxial input and output shafts, and a coplanar reverted gear train loop coupled to the input and output shaft through a rotating clutch. The coplanar reverted gear train loop comprises a toothed section disposed around the input shaft, a first ring gear disposed around the toothed section, a second ring gear disposed around the first ring gear, and an eccentric mounting ring for maintaining the first ring gear eccentric to the toothed section. The second ring gear is coupled to the output shaft and is coaxial to the toothed section. The rotating clutch is coupled between the input shaft and the mounting ring. When the clutch engages the mounting ring, the mounting ring rotates with the input shaft, thereby providing a first gear ratio of unity. When the clutch is disengaged from the mounting ring, a pair of toothed rings secured to the transmission housing hold the mounting ring against rotation, thereby providing a second gear ratio. Since the transmission does not require multiple gears to be mounted on a common shaft, the design reduces the alignment problems associated with the conventional variable-ratio gear box.
Gear changes are effected by providing the inner end of the input shaft with a series of axially extending teeth for slidably engaging with a similarly toothed hub carried on the input shaft. The hub is coupled to ratchet-toothed cylinder which engages a similarly-toothed cylinder coupled to the driving means. The rotating clutch is provided with actuation arms which engage a cam formed on the input shaft. With little or no load applied to the output shaft, the clutch engages the mounting ring to thereby produce the first gear ratio. However, when the load on the transmission increases, the ratchet teeth force the input shaft to be axially displaced. As a result, the cam applies pressure to the actuation arm, thereby disengaging the clutch from the mounting ring and producing the second gear ratio.
The transmission taught by Roeder includes a series of cascaded coplanar reverted gear train clutch assemblies for providing a plurality of gear ratios. However, as gear changes are affected by axial displacement of the respective input shafts, the transmission is only capable of producing gear ratios in which either all of the coplanar gear loops rotate with their respective input shaft, or one of the coplanar gear loops rotates with its respective input shaft and the mounting rings of all the preceding coplanar gear loops are held against rotation. Therefore, the total number of gear ratios available is limited. Furthermore, manufacturing and assembly costs are unnecessarily increased since the transmission requires both a rotating clutch, and toothed rings secured to the transmission housing. Accordingly, there remains a need for a variable ratio gear box which reduces the alignment problems associated with the prior art without limiting the number of gear ratios available and without unnecessarily increasing manufacturing and assembly costs.
Attempts have also been made to produce gear elements having an increased contact ratio between congruent teeth flanks. For example, Rennerfelt (U.S. Pat. No. 5,030,184) teaches an eccentric gear comprising a fixed spur gear having internal teeth, an eccentric cam secured to the input shaft of the eccentric gear, and an eccentric gear wheel rotatably coupled to the eccentric cam and having teeth engaging the internal teeth of the fixed spur gear. The teeth of the eccentric gear wheel and the teeth of the fixed spur gear are corrected by profile shifting to prevent collisions between the teeth and by stubbing to prevent collisions between the lands. The patentee states that the described profile shifting and stubbing increases the number of teeth in mesh. However, the patentee also acknowledges that, in theory, only one tooth in each loading direction will simultaneously be in mesh and that, in practice, one can achieve a greater number of teeth in mesh only by relying on the resilient nature of the teeth. However, such a practice can only increase the stress on each tooth. Accordingly, there remains a need for gear box having gears manufactured with teeth shaped to increase the contact ratio between congruent teeth flanks so as to reduce contact stress on the teeth bearing surfaces.
It is an object of the present invention to provide a variable ratio gear box having reduced sensitivity to gear alignment and having a greater number of available gear ratios in comparison with prior art gear boxes. It is also an object of the present invention to provide a variable ratio gear box having gears manufactured with teeth shaped to have an increased contact ratio between congruent teeth flanks.
In accordance with these objects, in a first embodiment of the invention there is provided a variable-ratio gear assembly comprising torque input means, torque output means, at least two intercoupled independently-variable coplanar reverted gear train loops coupled to the torque input means and the torque output means, and a clutch for varying a velocity ratio of the coplanar reverted gear train loops. Each coplanar reverted gear train loop comprises a torque input member, a torque output member and a reactive member. The clutch is coupled to the coplanar reverted gear train loops for independently switching at least two of the members from coupling to one of a rotational reference, the torque input means and the torque output means to coupling to another of the rotational reference, the torque input means and the torque output means.
In a second embodiment of the invention, there is provided a variable-ratio gear assembly comprising a coplanar reverted gear train loop including a primary torque input member, a primary torque output member, a primary torque reactive member, and an auxiliary torque member; and a clutch for varying a velocity ratio of the gear loop. The auxiliary torque member is one of a secondary torque input member, a secondary torque output member and a secondary reactive member. The clutch is coupled to the gear loop for switching the primary reactive member between coupling to one of a rotational reference, the primary torque input member and the primary torque output member to coupling to another of the rotational reference, the primary torque input member and the primary torque output member.
In one implementation of the second embodiment, the coplanar reverted gear train loop comprises a pinion gear, a pair of independent annular gears disposed around the pinion gear and being coaxial thereto, and a cluster gear member comprising a ring gear having an inner surface engaging the pinion gear and a pair of outer surfaces each engaging a respective one of the annular gears. In another implementation, the coplanar gear set comprises a pair of independent pinion gears, an annular gear disposed around the pinion gears and being coaxial thereto, and a cluster gear member comprising a ring gear having a pair of inner surfaces each engaging a respective one of the pinion gears and an outer surface engaging the annular gear.
In a third embodiment of the invention, there is provided a variable-ratio gear assembly comprising torque input means, torque output means, at least two independently-variable coplanar reverted gear train loops, and a clutch for varying a velocity ratio of the gear loops. A first of the coplanar reverted gear train loops comprises a first torque input member, a first torque output member and a first reactive member, while a second of the coplanar reverted gear train loops comprises a second torque input member, a second torque output member and a second reactive member. The first torque input member is coupled to the torque input means, the first torque output member is coupled to the torque output means, and the first reactive member is coupled to the second torque input member. The clutch is coupled to the second coplanar reverted gear train loop for selectively switching at least one of the second reactive member and the second torque output member from coupling to one of a rotational reference, the torque input means and the torque output means to coupling to another of the rotational reference, the torque input means and the torque output means.
In one implementation of the third embodiment, the clutch comprises first clutch means coupled to the second reactive member for switching the second reactive member from coupling to one of the rotational reference and the torque input means to another of the rotational reference and the torque input means, and second clutch means coupled to the second output member for switching the second reactive member from coupling to one of the rotational reference and the torque output means to another of the rotational reference and the torque output means.
In another implementation, the clutch comprises first clutch means coupled to the second reactive member and second clutch means coupled to the second output member for switching each said second reactive member and said second output member from coupling to one of the rotational reference and the torque input means to coupling to another of the rotational reference and the torque input means.
In yet another implementation, the clutch comprises first clutch means coupled to the second reactive member for coupling the second reactive member to any of the rotational reference, the torque input means and the torque output means, and second clutch means coupled to the second output member for switching the second output member from coupling to one of the rotational reference and the torque input means to coupling to another of the rotational reference and the torque input means.
For improved contact between gear teeth, each coplanar gear assembly in each embodiment preferably comprises an external gear including a first set of teeth and a first addendum circle; and an internal gear coplanar with the external gear and including a second set of teeth meshing with the first teeth set. The shape of an addendum flank of each first tooth and the shape of an addendum flank of each corresponding second tooth are defined by a curve of congruency passing between the addendum circles and extending between a point of intersection of the addendum circles and a pitch point of the gears.
Furthermore, to reduce manufacturing and assembly costs and to simplify operation of the gear assembly, the clutch in each embodiment comprises a binary clutch including a pair of clutch members, each clutch member including a set of concentric grooves disposed in a respective inner face thereof, one of the clutch members being adapted for coupling to one of the torque input means and the torque output means, and another of the clutch members being adapted for coupling to the other of the torque input means and the torque output means. An interactive member is disposed between the clutch members. The interactive member includes a pair of opposite surfaces and is adapted for coupling to a reactive member. A first of the surfaces includes a plurality of concentric grooves for meshing with one of the groove sets, and a second of the surfaces includes a plurality of concentric grooves for meshing with another of the groove sets. A clutch actuator is coupled to the interactive member for moving the interactive member between a first position engaging a first of the clutch members and a second position engaging a second of the clutch members.